The aim of this research program is to identify chemical alterations produced by ionizing radiation in DNA. The information being sought will contribute to determining the mechanism by which the bases are damaged and sugar-phosphate backbone is broken. The approach to this problem is to identify, by ESR and ENDOR, free radical products and reactions in single crystals of DNA model compounds. One of the specific objectives is to determine a comprehensive reaction scheme accounting for all major free radical products in both 3'cytidylic acid and 5' deoxycytidylic acid, X-irradiated and observed in the temperature range between 10K and 300K. Also under study are base complexes such as barbital: imidazole single crystals.